ADM8845EB-EVALZ Analog Devices Inc, ADM8845EB-EVALZ Datasheet - Page 10

• Manufacturer Part Number: ADM8845EB-EVALZ
• Description: BOARD EVALUATION FOR ADM8845
• Manufacturer: Analog Devices Inc

Specifications of ADM8845EB-EVALZ

• Current - Output / Channel: 20mA
• Outputs And Type: 6, Non-Isolated
• Voltage - Output: 5.5 V
• Features: Charge Pump, Dimmable
• Voltage - Input: 2.6 ~ 5.5V
• Utilized Ic / Part: ADM8845
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

ADM8845
THEORY OF OPERATION
The ADM8845 charge pump driver for LCD white LED back-
lights implements a multiple gain charge pump (1×, 1.5×, 2×) to
maintain the correct voltage on the anodes of the LEDs over a
2.6 V to 5.5 V (Li-Ion) input supply voltage. The charge pump
automatically switches between 1×/1.5×/2× modes, based on
the input voltage, to maintain sufficient drive for the LED anodes,
with V
tion of the charge pump output voltage for supply voltages up to
5.5 V. The six LEDs of the ADM8845 are arranged into two
groups, main and sub. The main display can have up to four
LEDs, FB1 to FB4, and the sub display can have one or two
LEDs, FB5 and FB6 (see Figure 23). Two digital input control
pins, CTRL1 and CTRL2, control the shutdown operation and
the brightness of the main and sub displays (see Table 4).
Table 4. Shutdown Truth Table
CTRL1
0
0
1
1
CC
input voltages as low as 2.6 V. It also includes regula-
CTRL2
0
1
0
1
4.7µF
R
CTRL1
CTRL2
SET
C4
I
V
SET
CC
LED Shutdown Operation
Sub Display Off/Main Display Off
Sub Display Off/Main Display On
Sub Display On/Main Display Off
Sub Display On/Main Display On
ADM8845
CURRENT
CONTROL
CIRCUIT
CONTROL
LED
LOGIC
CURRENT CONTROLLED SINKS
V
1µF
C1
1×/1.5×/2× MODE
CHARGE PUMP
REF
GND
OSC
1µF
C2
Figure 23. Functional Block Diagram
Rev. C | Page 10 of 20
An external resistor, R
GND. This resistor sets up a reference current, I
internally gained up by 120 within the ADM8845 to produce
the I
I
sinks to individually sense each LED current with a maximum
matching performance of 1%. This current matching perform-
ance ensures uniform brightness across a color display.
The ADM8845 lets the user control the brightness of the white
LEDs with a digital PWM signal applied to CTRL1 and/or
CTRL2. The duty cycle of the applied PWM signal determines
the brightness of the main and/or sub display backlight white
LEDs. The ADM8845 also allows the brightness of the white
LEDs to be controlled using a dc voltage (see Figure 22). Soft-
start circuitry limits the in-rush current flow at power-up. The
ADM8845 is fabricated using CMOS technology for minimal
power consumption and is packaged in a 16-lead lead frame
chip scale package.
SET
Figure 22. PWM Brightness Control Using a DC Voltage Applied to V
V
FB1
FB2
FB3
FB4
FB5
FB6
OUT
= 1.18 V/R
LED
C3
2.2µF
currents of up to 30 mA/LED (I
SET
MAIN
V
0V–2.5V
). The ADM8845 uses six individual current
BRIGHT
SET
, is connected between the I
R = 15kΩ
I
ADM8845
SET
R
SET
= 13.4kΩ
SUB
LED
= I
SET
SET
× 120 and
, which is
SET
pin and
BRIGHT